That's about the amount of weight of have been searching for in the 75-100kg range, I am in no rush, sure something is going to come my way.
I can defintly understand about driving it unloaded with the counter weights on, it feels iffy just normally not with more weight on the rear and less on the front, actually even with the bucket on it feels uncomfortable going forward up my driveway, its not that steep but just gives you that impression that it wants to stand on the rear door.
Mind you in reverse I would climb a tree in it. Back on course, on flat ground can't see a problem, just have to source these weights. even thought about casting some out of lead, I have a bit of that around, but you shouldn't handle it too much.
Cheers
Katoh